Noise control circuits



Patented Mar. 4, 1941 UNITED STATES PATENT OFFICE 2,233,761 NOISE CONTROL CIRCUITS Application April 30, 1938, SerialNo. 205,161

'2 Claims.

My present invention relates to noise control circuits, and more particularly to improved and eii'icient noise and static interference reduction circuits.

One of the main objects of my invention is to provide a grid-controlled, gaseous-dischargetube in a noise and static interference reduction network; the tube functioning to render the detection circuit of a modulated carrier receiver inefficient when electrical impulses exceeding a tolerable amplitude are received.

Another important object of my invention is to improve, and render highly reliable and efficient, noise reduction circuits by employinga grid-controlled, gas-filled tube as an automatic detection control device.

Another object of the invention is to use a gridcontrolled, gaseous-discharge tube as a. short-circuiting device for the detector load impedance of a receiver upon reception of impulses exceeding a desired amplitude.

Still another object of my invention is to use in a noise reduction system a gas-triode; the latter having a characteristic such that the control grid loses control when the plate voltage rises to a given value dependent on the grid voltage; a discharge taking place until the plate voltage-is reduced below the ionization potential. of the gas in the tube.

The novel features which I believe to be characteristic of my invention are'set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by 35 reference to the following description taken in connection with the drawing in which I have indicated diagrammatically circuit organizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows a noise reduction system embodying one form of the invention,

Fig. 2 illustrates the control characteristic of the noise control gas-triode,

Fig. 3 shows a modification of the invention.

Referring now to the accompanying drawing, wherein like reference characters in the figures designate similar circuit elements, there is shown a detector of modulated carrier waves. The detector may be, for example, of the diode type; the 50 signal input circuit l is connected to the anode and cathode of the diode 2. The diode anode is connected to the high potential side of input circuit I, while the cathode is at ground potential. The low potential side of circuit I is connected to ground through a pa'th including radio frequency choke coil 3 and diode load resistor 4. The condenser 5 bypasses radio frequency currents across the load resistor 4. The numeral 6 denotes a source of signals; the latter may be the I. F. (intermediate frequency) amplifier of a superheterodyne receiver, and in such case circuits 6 and I are each tuned to the operating I. F. value. The modulated carrier waves may be-those of the broadcast range of 500 to 1550 kc.

It is not believed necessary to show the networks preceding or following the diode detector;

those skilled in the art will understand the specific nature of such networks, as well as the fact that the detector may be used in a tuned radio frequency type of receiver if desired. Automatic volume control (A. V. C.) bias may be tapped from the load resistor 4 by connecting the signal grids of the amplifiers to the anode end of resistor 4. Audio voltage is taken from the latter by any desired type of adjustable tap l, and the audio voltage is fed to one or more audio amplifiers and a final reproducer.

In order to prevent reproduction of noise or static impulses, when the latter exceed a predetermined and tolerable amplitude, there is utilized a tube 8 of the 885 type. The latter is a gridcontrolled, gaseous-discharge tube of the heatercathode type. The tube 8 comprises a cathode 9, a grid l0 and a. plate II. The cathode 9 is connected to a point on resistor 4 to provide a biasing section 4'; the grid I0 is connected to the anode end of resistor 4 through a path including the negative voltage source [2 and resistor l3. The condenser I4 bypasses the audio voltage component from grid I0. The plate H is connected to ground through a path including resistor I5 and positive voltage source 16. The condenser l1 functions as a path for the application of audio voltage to plate H, and. together with the resistance in the circuit determines the time constant of the circuit which in turn determines the length of time the tube remains ionized and shorting the output of the receiver. This time constant may be adjusted for optimum performance under average noise conditions.

The gas-triode 8 has a control characteristic as shown in Fig. 2. A negative voltage on the control grid either maintains plate current cut-01f or promptly loses control, depending upon the plate voltage. After grid control is lost, it can be restored only by reducing the plate voltage be low the ionization potential of the gas in the tube. When the plate voltage reaches breakdown potential, the grid loses control at once. The tube has an extremely low 'de-ionization time. The

function of the grid I is to controlthe starting of plate current. For a given grid voltage, there is a particular plate voltage at which the discharge will just occur. This is shown in Fig. 2. Once the discharge has been set up, it cannot be further influenced by the grid, but it can be stopped by reducing the potential of plate ll below the ionization potential of the gas.

Since the rate of de-ionization is extremely rapid, a drop in plate voltage below the ionization potential causes instant current cut-off. This is a desirable characteristic for use in the noise control network shown in Fig. 1. The voltage across load resistor 4 has the polarity indicated; hence, the grid I0 is biased to prevent breakdown until the plate potential rises to a When the value sufficient to cause discharge. discharge in the tube 8 occurs, the tube acts as a short-circuit on the load resistor 4 for alternating current. The bias on grid in is adjusted so that no breakdown occurs from normal modulation voltages. Modulation voltages in excess of normal peak modulation voltage, such as that resulting from noise or static, discharges the tube 8 and reduces the receiver output to a very low value during the noise period. This low value will be such that the listener cannot hear the reproduction; that is, the receiver is rendered inoperative during the period of the noise impulses.

The plate II is maintained at a positive potential by virtue of two current sources. Cur.- rent source l6 applies a steady positive Voltage to the plate; the rectified carrier current flowing through resistor 4 supplies the remainder of the positive potential. The grid I0 is maintained at a negative potential by virtue of the voltage source l2, and the voltage developed across resistor section 4. The voltage source l2 provides suflicient negative bias to prevent the source I6 from discharging the tube 8. The additional negative bias across resistor section 4 increases in the same proportion as the rectified voltage applied to plate ll. Accordingly, once the tube 8 has its voltages adjusted, the operation of the tube is automatic. In other words, the negative bias developed across section 4 will automatically increase with the increase of rectified current magnitude, and this prevents the discharge between cathode 9 and plate ll, even though the same rectified current increase also increases the voltage of plate ll. However, when a sudden noise impulse exceeds modulation, the plate voltage applied to plate H by the rectified current increases sufficiently to break down the tube 8 and causes a short-circuiting discharge. In other words the magnitudes of voltage sources 4 and I6, as well as the magnitude of section 4, are so chosen that up to a predetermined percent modulation of the received carrier the tube 8 will not break down; whereas for electrical impulses corresponding to modulations in excess of the predetermined peak modulation, the tube will break down and prevent reproduction of the noise impulses. By choosing the normal peak modulation voltage to have a value corresponding to that of a 100% modulated wave, the tube will not operate to prevent reception for noise impulses which do not exceed 100% modulated carrier waves. The device in use causes no aural annoyance. The noise silencing tube produces fast gaps in the reception at noise points; over a continuous reception the gaps" are not discerned.

In Fig. 3 there is showna modification wherein the tube 8 has its plate to cathode path connected across the input circuit I of the diode detector 2. The plate of tube 8 is connected by the condenser I1 to the high potential side of the input circuit I, whereas the cathode is connected to the low potential side of the latter. The load resistor 4 is connected between the low potential side of the input circuit I and ground- 16. The cathode oftube 8 is connected to the junction of resistors l2 and Hi. The plate of tube 8 is connected to the resistor l6 through a radio frequency choke coil 20, and appropriate radio frequency by-pass condensers are connected from each of the cold electrodes of tube 8 to the cathode.

By .means of the adjustable grid bias source, or the adjustable plate voltage source, the tube 8 may be adjusted to operate, that is to discharge, on any, desired level of radio frequency input signal. If an electrical impulse, such as a noise impulse, of greater amplitude than the desired amplitude level appears, the gas tube 8 will discharge and effectively short-circuit the signal source for the duration of the noise pulse. It is not necessary to describe this modification of Fig .3 in any further detail, since it operates substantially in the same manner as described in the arrangement shown in Fig. 1. Essentially, in this modification, the tube 8 functions to short-circuit the input circuit l upon the impression on the detector 2 of noise impulses which exceed a predetermined peak modulated carrier amplitude.

While I have indicated and described several systems for carrying my invention into eifect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made Without departing from the scope of my invention as set forth inthe appended claims.

What I claim is:

1. In combination with a source of modulated carrier-waves and a diode-detector having a load resistor, means deriving an audio voltage from the load, a gas-triode provided with a characteristic such that a discharge occurs between plate and cathode when the plate voltage overcomes a predetermined control grid bias, means connecting the plate to cathode discharge path of the triode in shunt with the load resistor thereby to short-circuit the latter when a discharge occurs, capacity means connected between said load resistor and plate for applying said audio voltage to said plate, and means connecting the triode grid to a point on the load resistor which becomes increasingly negative as the modulated waves increase in intensity.-

. 2. In combination with a source of modulated carrier waves and a diode detector having a load resistor, means deriving an audio voltage from the load, a gas-triode provided with a characteristic such that a discharge occurs between plate and cathode when'the. plate voltage overcomes a predetermined control grid bias, means connecting the plate to cathode discharge path of the triode in shunt with the load resistor thereby to 'short-circuit'the later when a dispendent direct current voltage sources connected to the plate and grid of the triocle to estabcharge occurs, a condenser connected between said plate and the load resistor for applying said audio voltage to said plate, means connecting the triode grid to a point on the load resistor which becomes increasingly negative as the modulated waves increase in intensity, and indelish a desired minimum amplitude level at which I said discharge occurs. 

